In the coating of metals, protection from corrosion, i.e., the function of preserving their value, holds an equally prominent place with the decorative function. Protection from corrosion includes protection from environmental influences, particularly the influences of weather, water and aggressive media (chemicals). To perform this function, the coating must be resistant to influences of the environment. Moreover, the bond between coating and substrate, i.e., adhesion of the coating to the metallic substrate to be protected, must be as good as possible.
In recognition of these relationships, unsaturated polyesters have been used as additive binders to improve the adhesion of the coatings based on conventional binders. One group of state of the art polyesters is characterized by the absence of isophthalic acid in the acid component and by the presence of industrial mixtures of Formula I diols in the alcohol component of the monomer mixture on which the polyesters are based (DE-PS No. 953 117). ##STR1##
Experience has revealed a need to improve the adhesion of coatings produced with state of the art additive binders, especially with respect to such critical substrates as galvanized steel.